A. Human Tissue Plasminogen Activator
The fibrinolytic system is in a dynamic equilibrium with the coagulation system, maintaining an intact, patent vascular bed. The coagulation system deposits fibrin as a matrix serving to restore a hemostatic condition. The fibrinolytic system removes the fibrin network after the hemostatic condition is achieved. The fibrinolytic process is brought about by the proteolytic enzyme plasmin that is generated from a plasma protein precursor plasminogen. Plasminogen is converted to plasmin through activation by an activator.
Currently, two activators are commercially available, streptokinase and urokinase. Both are indicated for the treatment of acute vascular diseases such as myocardial infarct, stroke, pulmonary embolism, deep vein thrombosis, peripheral arterial occlusion and other venous thromboses. Collectively, these diseases account for major health hazards and risks.
The underlying etiological basis for these diseases points to either a partial, or in severe cases, total occlusion of a blood vessel by a blood clot--thrombus or thromboembolus. Traditional anticoagulant therapy, as with heparin and coumarin, does nothing to directly enhance dissolution of thrombi or thromboemboli. The thrombolytic agents referred to earlier, streptokinase and urokinase, have enjoyed practical and effective use. However, each has severe limitations. Neither has a high affinity for fibrin; consequently, both activate circulating and fibrin-bound plasminogen relatively indiscriminately. The plasmin formed in circulating blood is neutralized rather quickly and lost for useful thrombolysis. Residual plasmin will degrade several clotting factor proteins, for example, fibrinogen, Factor V and Factor VIII, causing a hemorrhagic potential. In addition, streptokinase is strongly antigenic and patients with high antibody titers respond inefficiently to treatment and cannot remain on continuous treatment. Urokinase therapy is expensive, owing to its involved isolation from human urine or tissue culture, and it, therefore, is not generally accepted in clinical practice. Urokinase has been the subject of numerous investigations--See, for example, references 1-6.
So-called plasminogen activators have been isolated from various human tissue, e.g., uterine tissue, blood, serum--see generally references 7-11 and from cell culture (reference 94). Compositions thereof have also been described--see references 12, 13. See also references 14-18. The plasminogen activators derived from these sources have been classified into two major groups: urokinase-type plasminogen activators (u-PA) and tissue-type plasminogen activators (t-PA) based on differences in their immunological properties. (The abbreviations t-PA and u-PA are those proposed at the XXVIII Meeting of the International Committee on Thrombosis and Hemostasis, Bergamo, Italy, July 27, 1982.)
Recently, a human melanoma line has been identified which secretes t-PA. Characterization of this melanoma plasminogen activator has shown it to be indistinguishable both immunologically and in amino acid composition from the plasminogen activator isolated from normal human tissue (Reference 19, 88).
The product was isolated in relatively pure form, characterized and found to be a highly active fibrinolytic agent (20).
Several studies (e.g. References 95 to 98) which used t-PA purified from the melanoma cell line have demonstrated its higher affinity for fibrin, compared with urokinase type plasminogen activators. More intensive investigation of human t-PA as a potential thrombolytic agent has, however, been hampered by its extremely low concentration in blood, tissue extracts, vessel perfusates and cell cultures.
It was perceived that the application of recombinant DNA and associated technologies would be a most effective way of providing the requisite large quantities of high quality human tissue-type plasminogen activator (earlier referred to as human plasminogen activator), essentially free of other human protein. Such materials would probably exhibit bioactivity admitting of their use clinically in the treatment of various cardiovascular conditions or diseases.
B. Recombinant DNA Technology
Recombinant DNA technology has reached the age of some sophistication. Molecular biologists are able to recombine various DNA sequences with some facility, creating new DNA entities capable of producing copious amounts of exogenous protein product in transformed microbes and cell cultures. The general means and methods are in hand for the in vitro ligation of various blunt ended or "sticky" ended fragments of DNA, producing potent expression vehicles useful in transforming particular organisms, thus directing their efficient synthesis of desired exogenous product. However, on an individual product basis, the pathway remains somewhat tortuous and the science has not advanced to a stage where regular predictions of success can be made. Indeed, those who portend successful results without the underlying experimental basis, do so with considerable risk of inoperability.
DNA recombination of the essential elements, i.e., an origin of replication, one or more phenotypic selection characteristics, an expression promoter, heterologous gene insert and remaining vector, generally is performed outside the host cell. The resulting recombinant replicable expression vehicle, or plasmid, is introduced into cells by transformation and large quantities of the recombinant vehicle obtained by growing the transformant. Where the gene is properly inserted with reference to portions which govern the transcription and translation of the encoded DNA message, the resulting expression vehicle is useful to actually produce the polypeptide sequence for which the inserted gene codes, a process referred to as expression. The resulting product may be obtained by lysing, if necessary, the host cell, in microbial systems, and recovering the product by appropriate purification from other proteins.
In practice, through the use of recombinant DNA technology, one can express entirely heterologous polypeptides--so-called direct expression--or alternatively may express a heterologous polypeptide fused to a portion of the amino acid sequence of a homologous polypeptide. In the latter cases, the intended bioactive product is sometimes rendered bioinactive within the fused, homologous/heterologous polypeptide until it is cleaved in an extracellular environment. See references (21) and (22).
Similarly, the art of cell or tissue cultures for studying genetics and cell physiology is well established. Means and methods are in hand for maintaining permanent cell lines, prepared by successive serial transfers from isolate normal cells. For use in research, such cell lines are maintained on a solid support in liquid medium, or by growth in suspension containing support nutriments. Scale-up for large preparations seems to pose only mechanical problems. For further background, attention is directed to references (23) and (24).
Likewise, protein biochemistry is a useful, indeed necessary, adjunct in biotechnology. Cells producing the desired protein also produce hundreds of other proteins, endogenous products of the cell's metabolism. These contaminating proteins, as well as other compounds, if not removed from the desired protein, could prove toxic if administered to an animal or human in the course of therapeutic treatment with desired protein. Hence, the techniques of protein biochemistry come to bear, allowing the design of separation procedures suitable for the particular system under consideration and providing a homogeneous product safe for intended use. Protein biochemistry also proves the identity of the desired product, characterizing it and ensuring that the cells have produced it faithfully with no alterations or mutations. This branch of science is also involved in the design of bioassays, stability studies and other procedures necessary to apply before successful clinical studies and marketing can take place.